The present invention relates generally to the field of lasers. More specifically, this invention relates to a method and laser apparatus for producing a pulsed output beam of laser radiation having a long effective pulse duration.
Facial telangiectasia (i.e., abnormal, dilated blood vessels) is a somewhat common occurrence in individuals with fair skin. Individuals having facial telangiectasia often seek cosmetic treatment for these vessels. The flashlamp-excited pulsed dye laser is an established device for the treatment of various cutaneous vascular lesions including facial telangiectasia. When treated with such a dye laser, most vessels can be cleared with a single treatment, and nearly all vessels can be cleared within three treatments.
Flashlamp-excited pulsed dye lasers used for treatment of facial telangiectasia (and various cutaneous vascular lesions) typically operate at a wavelength between 585 and 600 nanometers (nm) and a pulse duration of 450 microseconds (xcexcsec) to 1.5 milliseconds (msec). One of the treatment side effects particularly bothersome to patients is acute post-treatment purpura (i.e., superficial hemorrhage characterized by blue/black discoloration). Purpura is always bothersome, particularly when it appears on the face. Purpura can take one to three weeks to fully resolve and is often quite disruptive in a person""s work and social schedules. The acceptance of dye laser treatment would be considerably enhanced if purpura could be minimized or eliminated.
The cause for purpura in pulsed dye laser treatment of vascular lesions is generally thought to be caused by the extravasation of blood components from the rupture of the skin""s microvasculature, due to the rapid vessel heating produced by the relatively short pulse duration of the laser. Researchers have conjectured that if the pulse duration of the dye laser could be lengthened to a duration that is short compared to the thermal relaxation time of the abnormal vessels but long compared with the thermal relaxation time for the normal microvasculature, then the rupture of these vessels could be minimized or avoided. (Anderson, R. R. xe2x80x9cLaser-Tissue Interactions,xe2x80x9d Goldman, M. P., Fitzpatrick, R. E., Cutaneous Laser Surgery (St. Louis, Mosby, 1994), pp. 9-11.)
Various lasers currently used to treat facial telangiectasia generate a continuous pulse having a long pulse duration (i.e., greater than 10 msec). For example, the Aura(trademark) laser (manufactured by Laserscope, Inc. of San Jose, Calif.) and the Versapulse(trademark) laser (manufactured by Coherent, Inc., Medical Group, of Santa Clara, Calif.) are both solid-state lasers capable of generating output beams having a 10-20 msec pulse duration. Little or no purpura has been observed using these lasers. However, the depth of penetration of the output beam from these solid-state lasers is less than that of a pulsed dye laser beam due to the shorter wavelength (i.e., 532 nm). There is also stronger melanin absorption in the skin at this shorter wavelength. Consequently, the efficacy in treating deeper and larger telangiectasias is reduced when using one of these solid-state lasers.
The difficulty in forming a continuous output pulse having a long pulse duration with a flashlamp-excited dye laser is that accumulation of dye molecules in metastable electronic states, thermal distortions in the liquid dye medium (i.e., the gain medium) and photodegradation of the dye all can cause the laser action (i.e. lasing) to self-terminate. This self-termination occurs when the lasing threshold (i.e., the energy level at which laser action occurs) rises due to these conditions. With a constant amplitude excitation pulse, when the lasing threshold rises above the amplitude of the excitation pulse, lasing terminates.
U.S. Pat. No. 5,287,380 discloses a method of generating an excitation pulse with a ramped amplitude to extend the duration of the output beam of a flashlamp-excited pulsed dye laser. U.S. Pat. No. 5,598,426 discloses a ramping technique in which the amplitude of the excitation pulse changes in coordination with the lasing threshold to achieve an output beam pulse of extended duration. These patents describe methods that extend the pulse duration of the output beam to about 2 milliseconds. U.S. Pat. No. 5,746,735 describes optically combining the output beams of multiple lasers to obtain an output beam having a pulse duration of at least one millisecond. However, this technique requires 1 to 2 lasers for each millisecond of pulse duration.
A principle object of the present invention is to provide a flashlamp-excited pulsed dye laser having a long effective pulse duration (e.g., on the order of 5 to 40 milliseconds) so as to effectively treat facial telangiectasia (and various other cutaneous vascular lesions) with reduced post-treatment purpura.
The invention relates to an apparatus and method for generating an output beam of laser radiation having a long effective pulse duration (e.g., on the order of 5 to 40 milliseconds). For certain applications, a long effective laser pulse duration is desirable to allow thermal diffusion to occur during the time of laser energy deposition. An example is treating facial telangiectasia effectively while avoiding purpura. Another example is laser cutting and welding of metals and non-metals. While preferred embodiments include a flashlamp-excited pulsed dye laser, the principles of the invention can be advantageously applied to any laser with a dynamically increasing lasing threshold. Lasers with dynamically increasing lasing threshold include all lasers in which the lasing transition terminates in an energy level with appreciable thermal population. An example is the holmium laser operating near 2 micron wavelength. In this case, the lasing threshold increases during a laser pulse due to the increase in lower laser level population caused by the increase in temperature of the lasing medium. The principles of the present invention can also be advantageously used when the desired laser pulse duration is difficult to generate because the excitation energy required to maintain the laser above lasing threshold for the entire pulse is too great.
A laser incorporating the principles of the invention generates an output beam comprised of a series of micropulses of laser radiation. The series of micropulses constitute an output beam having a long xe2x80x9ceffectivexe2x80x9d pulse duration that has been shown to be effective in therapeutic treatment of cutaneous vascular legions while minimizing the undesired side effects of purpura or other hyper-, hypo-pigmentation abnormalities.
In one aspect, the invention features a method of generating a pulsed output beam of laser radiation having a long effective pulse duration in a flashlamp-excited dye laser. This method includes the steps of generating electronically a spaced series of excitation pulses, driving the flashlamp-excited dye laser with the series of excitation pulses, and producing an output beam comprised of a series of spaced micropulses of laser radiation which, in combination, have a long effective pulse duration.
In one embodiment, each pulse within the spaced series of excitation pulses has an amplitude that increases in coordination with an increasing lasing threshold for the flashlamp-excited dye laser. In another embodiment, a tuning element is adjusted in coordination with each pulse within the spaced series of excitation pulses to vary the wavelength of each micropulse. In another embodiment, the effective pulse duration of the output beam is at least 5 milliseconds. In another embodiment, each micropulse has a pulse duration of at least 10 microseconds. In another embodiment, each micropulse delivered to the skin has a fluence less than 5 j/cm2. In another embodiment, the series of micropulses delivered to the skin has a total fluence greater than 6 j/cm2. In another embodiment, each micropulse has an energy level that is less than half of the total energy of the series of micropulses. In another embodiment, the series of micropulses has a total energy greater than 1 Joule. In another embodiment, the output beam of laser radiation is directed at the skin of a patient to treat a cutaneous vascular lesion. In another embodiment, the pulse parameters are selected to minimize the formation of purpura. In another embodiment, the effective pulse duration of the output beam is between about 5 milliseconds and about 40 milliseconds.
In another aspect, the invention features a flashlamp-excited dye laser for producing a pulsed output beam having a long effective pulse duration. The laser includes a pulse forming module, a flashlamp and a lasing medium. The pulse forming module generates a series of spaced excitation pulses. The flashlamp is electrically connected to the pulse forming module and receives the series of spaced excitation pulses. The flashlamp uses the excitation pulses to produce a series of light pulses. The lasing medium can include a liquid dye mixture. The liquid dye medium receives the series of light pulses from the flashlamp and produces an output beam comprised of a series of spaced micropulses of laser radiation which, in combination, have a long effective pulse duration.
In one embodiment of the laser, each pulse within the spaced series of excitation pulses is generated with increasing amplitude in coordination with an increasing lasing threshold for the flashlamp-excited dye laser. In another embodiment, a tuning element is adjusted in coordination with each pulse within the spaced series of excitation pulses to vary the wavelength of each micropulse. In another embodiment, the effective pulse duration of the output beam is at least 5 milliseconds. In another embodiment, each micropulse has a pulse duration of at least 10 microseconds. In another embodiment, each micropulse has an energy level less than half of the total energy of the series of micropulses. In another embodiment, the series of micropulses has a total energy greater than 1 Joule. In another embodiment, the output beam of laser radiation is used to treat a cutaneous vascular lesion on a patient""s skin.